1. Field of the Invention
This invention relates to methods of making tubular heat exchangers, and particularly to a relatively simple and effective means for mechanically joining tubes to headers, expecially in heat exchangers of a high density, high performance type.
2. Description of the Prior Art
Heat exchangers in which many tubes are assembled in a densely packed relation find frequent use, especially in aircraft, where their compact construction and intensified heat transfer capabilities offer important advantages. The making of such heat exchangers poses certain problems, as in the mounting of tubes to supporting headers and in the sealing of the multitude of tube to header joints. In a typical example, a compact tubular heat exchanger consists of some 600 tubes mounted between headers six inches or less in diameter. Tubes have a diameter on the order of one-eighth inch and a wall thickness of 0.010 to 0.005. Tube accommodating holes in the header have diameters relatively closely to receive inserted tubes and are separated by ligaments of solid header material made as narrow as possible consistent with expected stresses and structural integrity. The tubes must be joined to the headers by means accomplishing both a seal and a bond.
Historically, the joining of tubes to headers, in a tubular heat exchanger, is done by swaging and rolling techniques. These are generally inapplicable to compact heat exchangers due to the small interior size of the tube and the likelihood of damage to the relatively thin tube and permanent stress that may be induced in relatively narrow header ligaments. Accordingly, the joining process commonly used is one of brazing. In a representative brazing process, a braze alloy in a slurry, foil or like form is introduced into each tube to header joint and a complete assembly comprising headers and mounted tubes is placed in a special furnace. There, in an inert atmosphere and under controlled conditions, the temperature of the assembly is raised to a determined value and then lowered. The braze alloy melts, flows to fill the joint in which it is contained, and then hardens.
As a joining technique, brazing has many advantages and is popularly and successfully used. It has some disadvantages, however, as in the difficulty of repair procedures with respect to individual joints. Also, under severe service conditions, fractures may occur at less strongly brazed joints, allowing leakage. These disadvantages become in some instances major considerations, leading in those instances to a demand for mechanically installed tubes. In meeting this demand, a prior art process has been developed in which hollow rivets or ferrules are inserted in tube ends and then expanded, using a special tool and axial motion. See U.S. Pat. No. 4,152,818, issued May 8, 1979. The prior art process achieves its desired ends. It is, however, a rather expensive procedure, and, since it places rivet-like devices in the flow path through the tubes, raises the pressure drop in the fluid flowing through the tubes. For some heat exchanger applications permitted pressure drop in the tube side fluid is a critical design consideration.